1. Field of the Invention
The present invention relates to an implementation for cooling and positioning prismatic battery cells.
2. Background Art
Batteries are used in a wide variety of electronic, electrical and mechanical devices. During charging and discharging, chemical reactions in batteries often cause the internal temperature to rise. The resulting higher temperature degrades battery performance and in extreme cases causes damage. This common battery overheating problem is also found in new battery applications such as Hybrid Electric Vehicles (HEV) and Electric Vehicles (EV). One type of battery used in EV and HEV is a Lithium-ion Polymer Battery (LiPB) cell. The LiPB cell is a new type of prismatic cell that can generate a high power output with a small battery size. Until recently, the use of LiPB has been largely confined to small portable electronic devices such as cellular phones. However, advances in technology has made new high-power LiPB an ideal candidate of EV and HEV applications. The new applications bring with them concerns of overheating because the higher power output generates more heat than previous applications in electronic devices. With its potentially high temperature application in EV and HEV, proper cooling and ventilation procedures must be implemented to address overheating.
In many common battery cooling implementations, air, water or some other liquid coolant is used as the cooling agent. Often air or a liquid coolant (e.g. water) is pumped around and between battery cells for cooling. Both air-based and liquid-based implementations have their drawbacks. With air cooling, the structural demand on the battery cells is high, especially in a battery module where cells are kept in compression. Air channels must be constructed around the battery cells to allow air to bring heat out of the system. The high spatial demand can limit the range of battery applications.
Liquid-based cooling is more effective than air cooling. For the same volume unit, liquid such as water can carry away more heat than air. Although the cooling fluid uses less volume, it weighs substantially more than air. The cooling fluid, along with re-circulation and plumbing fittings, adds significant weight to the overall battery module. The added weight often can be upward to 20% of the battery net weight. With the extra weight, the power output of the battery drops as power-to-weight ratio decreases.
An issue related to cooling is the positioning of battery cells. Often the cooling system also has to hold the battery cells in place. There are implementations which use plastic forming to contain battery cells. When plastic forming containers are used, there is a large weight gain and loss of cooling efficiency. This is because plastic, especially plastic thick enough to act as a structural member, is a very good insulator. For physically restraining cells in place, some implementations use the packaging to make up a module around the cells (e.g. for nickel-metal hydride NiMH batteries). The packaging is plastic and fastened to a structure that forms a battery pack.
Cooling Polymer Cells
Many of the aforementioned problems in battery cooling also exist in LiPB cells. First if a liquid-based implementation is used, the extra weight will add to the overall weight to the battery. This is a problem in HEV and EV where vehicle weight is to be minimized. However, if an air-based cooling implementation is used, a large amount of volume is needed to achieve adequate cooling efficiency. This negates a major advantage of LiPB cellxe2x80x94the high power-to-size ratio. Another problem that arises is the use of polymer in LiPB cells. Since a polymer is a plastic-like material, it acts an insulator and degrades cooling performance. The challenge, therefore, is to find a cooling implementation that is effective in dissipating heat, is compact, light in weight, and holds the battery cells in place. Such an implementation would also need to have low structural demand to minimize extra insulation.
The present invention relates to an implementation for cooling and positioning prismatic battery cells.
In one embodiment of the present invention, a thin cooling fin made of thermally conductive material is placed between prismatic battery cells. The cooling fin also acts as a structural constraint to hold the adjacent battery cell in place. Each cooling fin is larger than the cell itself and the area of the cooling fin in contact with the cell transfers heat to an area of the cooling fin that is not in contact with the cell. The area not in contact with the cell is corrugated to maximize surface area for heat dissipation. The corrugated area is direct contact with in a fluid stream (water, oil, etc.) or air so that heat is carried away by either conduction or convection. In another embodiment of the present invention, the cooling fins are constrained with other similar fins where an alternating (cell, cooling fin, cell, cooling fin, cell, etc.) geometry is obtained. Then all the fins are drawn together making a stronger single unit, keeping the cells in compression.
The present invention results in a much lighter and efficient structure than existing methods that solely rely on water or air circulation. The use of cooling fins to transfer heat from cells negates the need for air or water channels around each cell. This minimizes the volume of the overall battery module. Also the function of the cooling fins as structural constraint for the cells overcomes the problem of added insulation caused by extra plastic structural material used in existing implementations.